Unusual zwitterion of D,L-beta-carboxyaspartic acid: pKa and X-ray crystallographic measurements

An investigation of the acidic properties and molecular structure of the new natural amino acid beta-carboxyaspartic acid (Asa) is described. The four pKas of Asa were determined by using a microtitration technique and are 0.8 +/- 0.2, 2.5 +/- 0.1, 4.7 +/- 0.1, and 10.9 +/- 0.1. The three pKas of 5-hydantoinmalonic acid were similarly measured and are 1.85 +/- 0.05, 4.63 +/- 0.05, and 10.20 +/- 0.05. 5-Hydantoinmalonic acid was used as a model for Asa with peptide bonds. Asa crystallizes in the monoclinic space group Cc with four molecules per unit cell of dimensions a = 13.112 (3) A, b = 8.207 (3) A, and c = 7.292 (2) A and beta = 108.03 (2) degrees. The structure was solved by direct methods and refined to final values for the discrepancy indices of R = 0.029 and wR = 0.036. The two molecules of Asa are linked by a very strong hydrogen bond between one of the beta-carboxyls and the alpha-carboxyl group of an adjacent molecule. Analysis of the pKa data indicates that the predominate zwitterion in solution results from ionization of a beta-carboxyl group. The X-ray data indicate that in the solid state the negative charge of the zwitterion is distributed approximately equally between one of the beta-carboxyls and the alpha-carboxyl group.